Physics Flashcards
Focal spot sizes
- mammo
- general X ray
- CT
- Mammo: 0.1 mm and 0.3 mm
- general X ray: 0.6 and 1.2 mm
- CT: 0.6 and 1.2 cm
Things that improve (decrease) heel effect
- focus to film distance: larger FFD = less heel effect
- film size: smaller film = less heel effect
- larger anode angle = less heel effect
Beam intensity
- what is it?
- what is it measured in?
Beam intensity = number of X rays x energy
- Roentgens per minute (R/min)
- increases in proportion to kvP ^ 2
five factors that change the x ray spectrum
- mAs: quantity
- kVp: quantity and quality
- voltage generator: quantity and quality
- filtration: decreases quantity, increases quality
- target material: changes characteristic peaks; higher atomic number increases X-ray quality and quality
Effect of mA and kVp on focal spot
- high mA, low kVp = wider focal spot (blooming)
- higher kVp = smaller spot (thinning)
Probability of PE
directly proportional to Z ^3
inversely proportional to incident photon energy ^ 3
Attenuation of X ray in tissue depends on what 3 things?
- Z
- tissue density
- X ray beam Quality
3 things that increase scatter
- higher KVP
- larger field of view
- thicker part (person)
limiting factor for spatial resolution
- for film
- for CR
- for DR
- flim: size of grain of photographic chemical
- CR: size of laser used to read the phosphor palte
- DR: size of individual thermoluminescent transistor
factors causing geometric unsharpness
- larger focal spot
- increased object to detector distance
- decreased source to object distance
- more magnification
Magnification factor
source to image distance / source to object distance
SID/SOD
modulation transfer function (image)
= information recorded/ information available
Susceptibility artifact
- worse with longer TE
- worse with GRE sequences
Adjusting mAs
- for fat people?
- for children 6-12
- for children 0-25
double mAs for every 4 cm of patient tissue
- 50% for age 6-12
- 25% for age 0-5
ways to reduce scatter
- collimate
- compression
- lower kVp
- grid or air gap
